1. Technical Field of the Invention
The present invention relates in general to the mobile communications field and, in particular, to a method and system for improving the quality of transmissions in cellular communications systems.
2. Description of Related Art
Due to the rapid expansion of the wireless mobile market, and the increased need for wideband multimedia services, the available bandwidth has to be better utilized. A common strategy used for greater bandwidth utilization in Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA) systems is to reuse the frequencies allocated in the network, commonly referred to as xe2x80x9cfrequency reusexe2x80x9d. However, a problem with the frequency reuse strategy is that it creates interference between transmitters that needs to be counteracted.
One method used to reduce such interference is to control the power levels of the transmitted radio signals. For example, interference between different call connections using the same radio channel in a mobile radiotelephony system can be reduced by controlling the transmission power levels of the mobile stations (MSs) and base stations (BSs) in the system. The goal of such an approach is to ensure that only the transmission power necessary to maintain satisfactory call quality is used. When one communicator (MS or BS) thus controls its transmission power, the other communicators will experience lower interference from the first communicator, compared to the situation where it would be using maximum power. In turn, the system capacity can be increased (allegedly by about 70% compared to an unregulated system). In this regard, an important reason to keep the transmitted power of an MS as low as possible is to reduce the energy it consumes.
U.S. Pat. No. 4,485,486 to Webb et al. discloses a prior radiotelephone system in which power regulation is performed relatively infrequently, and is based on the questionable assumption that the disturbance level is more or less constant. The MS""s transmission power is only coarsely controlled, and the objective is to keep the received signal above the disturbance level. However, in practice, the disturbance level varies considerably in both time and place. Consequently, this algorithm is based on incorrect assumptions.
Most power control algorithms proposed to date, strive to balance the carrier-to-interference ratios (C/Is) on each channel so that every MS or BS achieves the same C/I. As disclosed in xe2x80x9cPerformance of Optimum Transmitter Power Control in Cellular Radio Systems,xe2x80x9d by J. Zander, IEEE Transactions on Vehicular Technology, 41(1):57-62, 1992, for every traffic scenario in a cellular communications system, there is a maximum C/I that can be obtained by all receivers at the same time. If all pertinent information were to be available at one place, a global (or centralized) power control approach is proposed that assigns the transmission power levels so that this maximum C/I is achieved. However, such an approach requires extensive signaling in the network. Therefore, it is more desirable to perform power control in a distributed fashion, wherein only local measurements are used. Such a requirement is fulfilled by the xe2x80x9cModified Distributed Balancing Algorithmxe2x80x9d (MDBA) disclosed in xe2x80x9cA Simple Distributed Autonomous Power Control Algorithm and its Convergence,xe2x80x9d by G. J. Foschini and Z. Miljanic, IEEE Transactions on Vehicular Technology, 42(4):641-646, 1993.
The problem associated with the above-described approach is to assign an appropriate target C/I that the power control algorithm can strive to achieve. If the target value is set too high, the transmission power levels might be increased to maximum levels determined by the physical limits (constraints) of the system, without achieving the specified target level. In that regard, an attempt to employ graceful degradation in such systems has been disclosed in U.S. Pat. No. 5,574,982 to Almgren et al., and xe2x80x9cPower Control in a Cellular System,xe2x80x9d by M. Almgren, H. Andersson, and K. Wallstedt, Proceedings of the 44th Vehicular Technology Conference, Stockholm, Sweden, May 1994. This approach was further refined in xe2x80x9cSoft Dropping Power Control,xe2x80x9d by R. D. Yates, S. Gupta, C. Rose, and S. Sohn, Proceedings of the 47th Vehicular Technology Conference, Phoenix, Ariz., May 1997.
In xe2x80x9cFrequency Hopping GSM,xe2x80x9d by C. Carneheim, S.-O. Jonsson, M. Ljungberg, M. Madfors, and J. Nxc3xa4slund, Proceedings of the 44th Vehicular Technology Conference, Stockholm, Sweden, May 1994, a different power control approach has been disclosed in which the transmission power levels are based on signal attenuation and C/I. The specific case where the output power levels have an upper limit, pmax, has been studied in xe2x80x9cConstrained Power Control,xe2x80x9d by S. Grandhi, J. Zander, and R. Yates, Wireless Personal Communications, 1:257-270, 1995, where the Distributed Constrained Power Control (DCPC) algorithm has been disclosed. Additional complexity arises considering that the transmission power levels are not only limited by hardware constraints but also quantized. Therefore, only a discrete set of power levels can be used. In any event, the above-described power control algorithms may be accompanied by a Dynamic Channel Allocation (DCA) algorithm which assigns appropriate channels in order to further decrease interference.
A major problem with the existing transmission power regulation approaches is that they are based on the assumption that the transmission quality is dependent only on the C/I. However, although this assumption may be true for analog systems, it is not true for digital systems. Instead, for digital systems, a transmission quality measure is needed that describes the effects of coding, frequency hopping, interleaving, etc., if these functions are being used. Consequently, it may be desirable to use such a quality measure to specify quality requirements instead of directly specifying the target C/I. Intuitively, one realizes that such an algorithm should be able to adapt to the specific interference circumstances at each receiver.
In xe2x80x9cImproved Quality Estimation for Use in Simulation of Wireless TDMA Systems,xe2x80x9d Proceedings of the 6th International Conference on Universal Personal Communications, by H. Olofsson, San Diego, USA, October 1997, transmission quality in TDMA systems is described in terms of Frame Erasure Rate (FER), which better reflects the actual quality perceived by the user. As such, use of the FER for transmission quality purposes is an appropriate choice for the Global System for Mobile Communications (GSM).
The C/I of a connection is not measurable directly. Instead, the measurement reports can comprise a Quality Indicator (QI) and a Received Signal Strength Indicator (RSSI). As such, both of these values are quantized, and a great deal of pertinent information is lost in the quantization process. The measurement reports may comprise other measures, but in the GSM, these values are used, and denoted as xe2x80x9cRXQUALxe2x80x9d and xe2x80x9cRXLEVxe2x80x9d, respectively. Consequently, the C/I used in the power control algorithm has to be estimated from these values. However, the approach used today is not very accurate, and thus it is desirable to extract as much pertinent information as possible from the measurement reports to increase this accuracy.
In a practical situation, there are physical limitations in the transmitters, receivers, and the network itself. The transmission power levels are not only limited but also quantized. Therefore, only a discrete set of power levels can be used. The signaling, and the signal strength and quality measurements involved take up considerable amounts of time, which result in significant time delays in the network. Some standards only allow this report and control signaling at certain time intervals, resulting in additional delays. The effects of these delays can cause oscillations in the transmission powers, if the controller parameters have not been properly chosen. As described in detail below, the present invention successfully resolves these problems.
A method and system for quality-based transmission power control in a cellular communications system are provided, whereby a network operator can specify the transmission quality requirements using a measurement that better reflects the actual quality perceived by the users. All transmitter power levels in the network can be controlled by identical power regulators, each of which can adapt to individual situations in order to achieve the specified quality. For example, in a GSM frequency-hopping network, the FER together with parameters estimated from the current traffic situation, are mapped onto a target C/I, which in turn, the power control algorithm strives to achieve. Consequently, the power regulators can adapt to the traffic situation experienced by each receiver.
An important technical advantage of the present invention is that the cellular system""s transmission quality requirements can be specified using a measure that better reflects the actual quality perceived by the users.
Another important technical advantage of the present invention is that an estimator is used to extract as much pertinent information as possible from the measurement reports, such as interference characteristics, path loss, the carrier, and the C/I.
Still another important technical advantage of the present invention is that the use of an estimator and quality mapper reduces the function of the power control algorithm to what it is good at, namely to assign transmitter power levels so that the (estimated) C/I will accurately track the (computed) target C/I. Thereby, the specified transmission quality will be achieved.
Yet another important technical advantage of the present invention is that it includes methods for choosing appropriate parameter values, in order to prevent instability and counteract the effects of time delays.
Yet another important technical advantage of the present invention is that the power control algorithm compensates for the effects of the physical constraints in the cellular system, on the quality of the transmission.
Still another important technical advantage of the present invention is that quality-based power control can be readily implemented in existing systems. The primary component to be updated is the software in the BSs, where the output power levels are computed. Consequently, the present invention can be implemented without affecting the existing technical standards, the MSs, radio interfaces, or BS transmitters, which is extremely important due to the great number of terminals in use today.